Hello I have $15 on my Apple Store and I was unable to purchase something on my iCloud

You have money in your Apple Account balance -> have $15 

See -> Add a payment method to your Apple ID – Apple Support (UK)

• if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.

Then you can make purchase in the App Store.

If you still need help, get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

There is something wrong that cannot be fixed at your end. Please Contact - Official Apple Support for further support on this issue